1. Field of the Invention
This invention relates generally to implantable cardiac stimulators, and more particularly to circuit modules encased within cardiac stimulators.
2. Description of the Related Art
The advent of implantable cardiac stimulation systems, such as pacemakers and defibrillators, has brought welcome relief to many patients suffering from various forms of cardiac arrhythmia. Conventional cardiac stimulator systems typically consist of a cardiac stimulator and one or more elongated leads. The cardiac stimulator may be a pacemaker, a defibrillator, a sensing instrument, or some combination thereof. The circuitry, batteries, and other components of the cardiac stimulator are ordinarily encased within a metallic housing commonly referred to as a "can." Most of the electronic components for the cardiac stimulator are mounted on a small circuit board commonly known as a multi-chip or hybrid module.
The proximal ends of the leads of the cardiac stimulator system are connected physically and electrically to the cardiac stimulator via a structure commonly known as a header. The distal end of the lead is implanted near the site requiring electrical stimulation or sensing. The leads function to carry electrical signals from the cardiac stimulator to the targeted tissue and signals from the targeted tissue back to the cardiac stimulator.
For most implantable cardiac stimulators, implantation requires an incision in the right or left pectoral region above the areola and formation of a pocket in the subcutaneous tissue by blunt dissection. The leads are then passed into the body to the sites requiring electrical stimulation, usually with the aid of a stylet. The proximal ends of the leads are then connected to the header of the cardiac stimulator and the cardiac stimulator is inserted through the incision and placed in the pocket. The incision is then closed by conventional suturing. The post-operative appearance of the implant area will depend to a large degree on the size of the cardiac stimulator.
A conventional multi-chip module consists of a number of electronic devices disposed on one or both sides of a flat insulating substrate. Depending upon the type of cardiac stimulator, the devices may be discrete devices, such as resistors and capacitors, or more highly integrated devices, such as power transistors, microprocessors, telemetry circuits, or induction coils for rechargeable storage devices. Depending on the manufacturer, the components may be packaged, unpackaged, or a combination of the two.
Surface mounting and chip-and-wire represent two common techniques for mounting both packaged and unpackaged components on a multi-chip module. For packaged parts, surface mounting involves coupling the package to the multi-chip module by soldering the pins of the package to metal traces on the multi-chip module. The package may be further secured by an adhesive. The package pins provide connectivity with the enclosed component via bonding wires protected by the package. In a variant of surface mounting known as flip-chip, an unpackaged part fabricated on a die, such as an application specific integrated circuit ("ASIC") or microprocessor, is mounted directly on the multi-chip module. The bond pads on the die are typically bump connected to metallization on the multi-chip module. Flip chip mounting technology has the advantage of potentially high packing density. However, for components that are traditionally supplied in packaged form, such as diodes and resistors, the footprints of the packages are relatively large and represent a limit on the achievable packing density for the multi-chip module.
In chip-and-wire mounting, unpackaged components are secured to the multi-chip module by an adhesive and connectivity with the multi-chip module is established by bonding wires connected between the bond pads of the component and metallization on the multi-chip module. Chip-and-wire mounting can achieve higher packing density than is possible in surface mounting for those types of components supplied in packages with large footprints. However, conventional chip-and-wire processing is generally incompatible with surface mount processing. The problem stems from the fact that the solder and solder flux used to mount a surface mount package may short or damage the tiny bonding wires connecting the bonding pads of a bare die to the multi-chip module substrate. Aside from incompatibility with surface mount processing, conventional chip-and-wire mounted components must be separately tested. Such component-by-component electrical testing adds to cost of the overall manufacturing process.
One method currently used in attempt to alleviate the incompatibility problem involves mounting bare die on one side of the multi-chip module substrate using chip-and-wire processing, and surface mounting other components on the opposite side of the multi-chip module substrate. A drawback associated with this method is that complex metallization must be fabricated on both sides of the multi-chip module substrate and the overall thickness of the multi-chip module is increased.
The present invention is directed to overcoming or reducing one or more of the foregoing disadvantages.